ACOG now says that women should get their first Pap smear, a screener for cervical cancer, at age 21, with follow-up tests every other year until they turn 30; women over 30 who have had three consecutive "normal" results can then wait three years between tests. The group points to studies showing that only 0.1 percent of cervical cancer cases occur in women under 21, but that does little to ease big fears that the killer disease could grow undetected between spaced-out exams.

The bottom line: "It's all about risk," says Celeste Robb-Nicholson, M.D., associate chief of general medicine at Massachusetts General Hospital. "A healthy woman who is HPV negative and is either not sexually active or has just one partner can be screened every three years. Women who have risk factors—such as smoking and having multiple partners—should be screened annually."